Pressure sensitive material

ABSTRACT

A polymeric material which exhibits an optically detectable response to changes in pressure. The material includes an elastomer selected from the group of a polyurethane, a polyacrylate, and a silicone, in combination with a photochemical system. The photochemical system may be in the form of an exciplex or a fluorescence resonance energy transfer. Both photochemical systems are reversible processes. Synthesis of the elastomer and the photochemical system produce a material which forms an excited charge transfer complex when subject to an increase in pressure and a less excited charge transfer complex as pressure is lowered.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to polymeric material useful in acquiringquantitative surface pressure measurements. More specifically, theinvention relates to synthesis of a nano-material which exhibits anoptically detectable response to changes in pressure.

2. Description of the Prior Art

Acquisition of global, surface pressure data by optical non-intrusivemethods has been sought after for many years. Techniques used for theacquisition of these data range from detection of Raman scattering tomaterials commonly called pressure sensitive paints. Traditionally,pressure sensitive paints consist of a host matrix in which one of avariety of chromophores is encapsulated. The host matrix is often apolymeric material such as polydimethylsiloxane (PDMS), but othermaterials such as sol-gels have been used. Typical chromophores usedhave included platinum octaethylporphyrin (PtOEP) and ruthenium-basedcomplexes. The functionality of these pressure sensitive paints dependson the dynamic quenching of the chromophore's luminescent emission byoxygen. In order for this dynamic quenching to be effective the hostmatrix must allow the diffusion of oxygen throughout the “paint” to thechromophores. One example of a prior art application requiring thediffusion of oxygen is U.S. Pat. No. 5,965,632 to Gouterman whichteaches the use of a pressure sensitive pain incorporating an acrylicand flouroarcrylic polymer binder. A pressure sensing dye is dissolvedor dispersed in the polymer matrix. The dyes illuminate in the presenceof molecular oxygen. Similarly, in a prior non-related application toKelley et al., the pressure sensitive material used has a host polymerand a fluorescent compound attached to the host polymer. The hostpolymer has a “rubber like” characteristic rather than a rubberyelastomer. In addition, Kelley et al. focuses on the use of polystyrenein place of a polyurethane and rubberized polymethacrylate because itdoes not contain oxygen. Accordingly, one of the limitation of the priorart pressure sensitive paints is the sensitivity to oxygen.

Dynamic quenching by oxygen follows an association known as theStern-Volmer relationship. This relationship between changes inluminescent emission intensity, I, and the local partial pressure ofoxygen, p_(o), is expressed as I_(o))/I=A+B(p/p_(o)) whereA=k_(o)/(k_(o)+k_(o)p_(o) and B=k_(o)p_(o)/k_(o)+k_(o)p_(o)). In theseequations I_(o) is the incident excitation light intensity, k_(o) is theintrinsic de-excitation rate in the absence of oxygen, k_(q) is thequenching rate due to collisions with oxygen and p is the localpressure. In addition, A+B=1. A typical plot of the relationship betweenchanges in luminescent emission intensity and local partial pressure ofoxygen is shown in FIG. 1. Under the conditions normally experiencedduring high-speed tests (e.g. supersonic), systems following theStern-Volmer relationship exhibit relatively large changes in emissionintensity for only small changes in pressure. However, the same systemsused for low-speed (e.g. atmospheric) tests exhibit only extremely smallchanges in emission intensity even for large changes in pressure. Thisis shown schematically in FIG. 2 which is a graph showing theStern-Volmer relationship between small changes in intensity and largechanges in pressure. In addition, systems following the Stern-Volmerrelationship exhibit decreasing emission intensity with increasingpressure. Accordingly, this results in lower signal to noise ratios withthe maximum signal to noise ratio at vacuum, or near vacuum, conditions.

Because these systems rely on oxygen quenching to vary emission lightintensity with changes in pressure, any perturbation to the host matrix'oxygen penmeability alters the pressure sensitive paint's performance.For example, variations in humidity and/or temperature affect pressuresensitive paint's performance. Unfortunately, even the oils normallyfound on human skin have been known to affect the performance of sometraditional pressure sensitive paint formulations making handling ofpainted test articles difficult. Accordingly, there is a need for apressure sensitive material that mitigates sensitivity to oxygen.

SUMMARY OF THE INVENTION

This invention comprises a nano-material adapted to exhibit an opticallydetectable response to changes in pressure.

In a first aspect of the invention a polymeric material for sensingpressure is provided. The material includes a polyurethane elastomerselected from the group of an aliphatic diisocyanate, a hydroxlterminated polyol, and a photochemical system modified to be a chainextending diol. In addition, the material includes an isocyanate tohydroxyl molar ratio ranging from about 1 to 2 and a molar ratio of thediol mix ranging from about 10:1 to about 1:2. The photochemical systemmay be an exciplex or a fluorescence resonance energy transfer (FRET).The aliphatic diisocyanate may be in the form of isophorone diisocyanateand diisocyanato hexainethylene. The hydroxyl terminated polyol may bein the form of polypropylene glycol or polytetramethylene glycol. Thepolyurethane elastomer is preferably adapted to form an excited chargetransfer complex when it is subject to an increase in pressure and aless excited charge transfer complex as pressure is lowered. The excitedcharge transfer preferably provides an optically detectable luminescentemission in response to a change in pressure. The polyurethane elastomermay include probes in the chain to measure deformation when subject topressure. The probes preferably report movement in the chain throughchanges in spectral emission. In a further embodiment, the polyurethaneelastomer may be fornulated into a solution to be applied to a secondarysurface, wherein the elastomer comprises from 3% to about 10% by weightof the solution. The solutions preferably enable application of thematerial to a secondary surface through a spraying apparatus.

In a second aspect of the invention, a polymeric material in the form ofan elastomer selected from the group of a polyacrylate and a solicone,in combination with a photochemical system is used for sensing pressure.The polyacrylate elastomer is selected from the group consisting of abutyl acrylate, and a methyl methacrylate. A percentage of the butylacrylate, methyl methacrylate, and silicon weight preferably ranges fromabout 20% to about 90%, and the photochemical system includes a dyemolecule range from about 1 milligram to about 100 milligrams dye per 10grams of polymer. The photochemical system is preferably an exciplex ora fluorescence resonance energy transfer. The exciplex moleculecombination may include anthracene and dimethylaniline, perylene anddimethylaniline, or pryene and perylene. The FRET donor-acceptor systemis preferebly Fluorescein donor and Rhodamine acceptor. The polyacrylateelastomer preferably comprises from about 3% to about 10% by weight ofthe solution. The solution may include solvents such as ethanol,methanol, isopropanol, methyl ethyl ketone, acetone and/or toluene. Thepurpose of the solvents is to preferably control properties such asevaporation rate, coating thickness, coating quality, and spectralresponse. The solution may be applied to a secondary surface through aspraying apparatus.

Other features and advantages of this invention will become apparentfrom the following detailed description of the presently preferredembodiment of the invention, taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art graph illustrating the relationship betweenchanges in luminescent emission intensity and local partial pressure ofoxygen.

FIG. 2 is a prior art graph illustrating the Stern-Volmer relationshipbetween small changes in intensity and large changes in pressure.

FIG. 3 is a graph illustrating of a typical spectral response of anexciplex forming system.

FIG. 4 is a graph illustrating the change in spectral response withchanges in pressure.

FIG. 5 is a prior art graph illustrating a ratiometric pressuresensitive paint response to changes in pressure.

FIG. 6 is graph illustrating a FRET emission spectra according to thepreferred embodiment of this invention, and is suggested for printing onthe first page of the issued patent.

DESCRIPTION OF THE PREFERRED EMBODIMENT Overview

The primary embodiment of this invention concerns the design, synthesis,and assembly of macromolecules on the nanoscale level. Fluorescentdistance probing molecules are copolymerized onto polymer chains duringpolymer synthesis. The choice of probes, ratio of probes, concentrationof the polymer, placement along the polymer chain, and the types ofsolvents used are parameters that are integral to performance of thematerial. The distance probes are used in this invention to measure thenano-deformation of a polymeric material as it is placed under load(pressure). As the material compresses or expands on the macro-scale,the polymer chains reorganize themselves in response to the load and theprobes report the movement. Accordingly, the movement is reported anddetected by the changing emission spectrum of the polymer.

Technical Details

1. Photochemical System

The are two forms of a photochemical systems used in this invention, anexcited state complex (exciplex) and fluorescence resonance energytransfer (FRET). Both photochemical systems are reversible. An exciplex(excited state complex) is the result of the formation of a chargetransfer complex between an excited state fluorophore and a quencher.FIG. 3 is a graphic illustration of a typical spectral response of anexciplex forming system. In exciplex formation, an excited statefluorophore such as anthracene or perylene is quenched by an aliphaticor aromatic amine (e.g.dimethylaniline). FIG. 4 is a graphicillustration of perylene emission data and exciplex emission data. Theexcited state fluorophore accepts an electron from the donating amine,and fluorescence from the exciplex is observed as a broad featurelesspeak red shifted from the fluorophore. Accordingly, the exciplex has afluorescence emission spectrum unique from the donor or acceptor.

The exciplex formation process is distance dependent. A criticalintermolecular acceptor to donor distance (˜2 Å) must be reached foremission of the complex to take place. The process is concentrationdependent in solution, as well as in a solid matrix. Accordingly, donorconcentrations, acceptor concentrations, and the acceptor to donorratios are parameters that influence the emission spectra.

FRET is an alternative distance dependent system from the exciplex. InFRET, transfer of excited state energy takes place from an initiallyexcited donor (D) to an acceptor (A). The donor and acceptor designationrefers to energy, as opposed to the exciplex system in which thenomenclature refers to electrons. It is required that the absorptionspectrum of the acceptor must overlap the fluorescence emission spectrumof the donor for FRET to occur. The intermolecular distances requiredfor FRET are in the order of 20 to 60 Å, which is advantageous forprobing movements of macromolecules. The energy transfer in FRET takesplace without the emission and reabsorption of photons, and is solelythe result of dipole-dipole interactions between donors and acceptors.One of the most common donor-acceptor systems in FRET is Fluorescein(Fl, donor) and Rhodamine B (Rh, acceptor). An example of the FRETemission spectra is shown in FIG. 6.

The Fluorescein and Rhodamine B system has potential as a distancedependent energy transfer system for pressure sensitive paint. Theexcitation wavelength that is commonly used in the Fluorescein andRhodamine B system is 470 nm, which is compatible with existing pressuresensitive paint systems. The emission wavelengths of Fluorescein andRhodamine B are far enough apart so that they can be optically isolatedduring signal detection. In FRET, the concentrations of constituentmolecules are much less than what is required in the exciplex system.During material design the luminophores can be copolymerized in lowweight percentages so as to not adversely alter the material properties.Accordingly, the FRET has some additional material properties advantagesover the exciplex.

2. Materials

The luminescent pressure sensor described herein is a coating based onpolymers such as polyurethanes, polyacrylates, and silicone. Specialtymonomers which are specific to the exciplex or FRET systems arecopolymerized with the coating during polymer synthesis. The materialschosen for this invention are elastomeric, meaning that they possessrubber-like properties and are capable of experiencing large andreversible elastic deformations. Accordingly, the elastomeric propertiesof the material in combination with thc reversible photochemical processform an excited charge transfer complex or FRET when the material issubject to an increase in pressure and a less excited charge transfercomplex or FRET as pressure is lowered.

Having the fluorescent monomers directly attached to the elastomerchains in this invention have the following significant advantages: 1)no dyes are lost during sensor use due to vaporization, sublimation, ormigration to the environment, 2) aggregation of the dyes are prevented,and 3) the material properties together with the donor-acceptor ratiodetermine the sensitivity to pressure, and response of the luminescentpressure sensor.

The composition of the polyurethane elastomers for pressure sensitivecoatings include, but are not limited to an aliphatic diisocyanate suchas isophorone diisocyanate (IPDI) or diisocyanatohexamethylene (HDI), ahydroxyl terminated polyol such as polypropylene glycol (PPG) orpolytetramethylene glycol (PTMO or PTMEG), and an exciplex or FRETparticipating molecule modified to be a chain extending diol. Anotherchain extender such as butane diol may be part of the polyurethanecomposition. Properties of the urethane coating (i.e. modulus, adhesion,solution viscosity, etc.) can be modified by adjusting the componenttype, their amount, and their weight ratios in the polymer synthesis.

In the present invention, the total isocyanate to hydroxyl molar ratio(NCO:OH) ranges from 1 to 2. Ratio values close to 1 produce linearelastomers, and values approaching 2 results in prepolymers capable ofmoisture curing into crosslinked coatings. The molar ratio of the diolmix (chain extender to polyol) can range from 10:1 to 1:2 in thisinvention.

The composition of the polyacrylates for pressure sensitive coatingsinclude, but are not limited to, butyl acrylate (BA), methylmethacrylate (MMA), and exciplex or FRET participating moleculesmodified for acrylate polymerization. The physical properties of thepolyacrylate coating can be tailored by adjusting the weight ratio ofbutyl acrylate to methyl methacrylate or exciplex in the polymersynthesis. Typical butyl acrylate weight percents of butyl acrylate inthis invention range from 20% to 90%. The remaining weight fraction maybe made up of methyl methacrylate or exciplex forming acrylate monomer.In a polyacrylate composition using the FRET in place of the exciplex,only a minute amount of FRET forming acrylate dye is needed in theacrylate synthesis (on the order of 1 milligram to about 100 milligramsof dye per 10 grams polymer).

The composition of the silicones for pressure sensitive coatingsinclude, but are not limited to, GE silicone TSE-399c and a highviscosity silicon sealant, and exciplex or FRET participating moleculesmodified for silicon polymerization. The physical properties of thesilicone coating can be tailored by adjusting the weight ratio of thesilicones and the photochemical system.

EXAMPLES

Polyurethane Pressure Sensitive Material Example Synthesis:

A monomer mix of PPG (molecular weight: 2000 grams/mole; 8 grams, 0.004moles) and dimethylaniline diol (DMAD) (molecular weight: 209.29grams/mole; 1.672 grams, 0.008 moles) was added to a 125 ml 3 neck flaskwith 40 uL of dibutyl tin dilaurate (DBTDL) as catalyst. The flask wasfitted with a condenser, an inlet for dry nitrogen, and an additionfunnel. The flask was immersed in an oil bath and the contents wereplaced under a blanket of dry nitrogen. Anhydrous tetrahydrofuran (THF,20 mL) was added through the addition funnel, and the flask was slowlyheated to 70° C. At a reaction temperature of 70° C., isophoronediisocyanate (IPDI) (molecular weight: 222.29 grams/mole; 2.67 g, 0.0012moles) and 5 mL of anyhrdrous tetrahydrofuran (THF) were added slowlythrough the addition funnel. The reaction mix was stirred for a total of5 hours then cooled. The solid elastomeric product weighed approximately12 grams and was obtained by removing the solvent under reducedpressure.

Polyacrylate Pressure Sensitive Material Example Synthesis:

A monomer mix of butyl acrylate (BA) and methyl methacrylate (MMA) in70:30 weight ratio (7 grams BA, 3 grams MMA) was placed in a 3 neck 125mL flask along with dibenzoyl peroxide (BPO, 0.5% by weight, 50milligrams), Rhodamine B acrylate monomer (0.8 milligrams), and 38 mL ofethanol. The flask was fitted with a condenser and dry nitrogen inletthen placed in an oil bath. The reaction contents were slowly heated to90° C. and the temperature was maintained for the course of thereaction. Total reaction time was 48 hours. The solid elastomericproduct weighed approximately 10 grams and was obtained by removing thesolvent under reduced pressure.

3. Materials Processing

Pressure sensitive materials based on polyurethanes in the presentinvention are formulated into solutions capable of being sprayed. Thereaction mixture is diluted to a solution with a solid content of 3% to10% (weight/volume) using solvents including tetrahydrofuran, toluene,isopropanol, methanol, and methyl ethyl ketone. The solution may includesome or all of the above listed solvents in various ratios in theformulation to control the evaporation rate, coating thickness, andcoating quality. The formulation may include the addition of plasticizerto control the coating properties and sensor response.

The formulation of acrylate or silicon based pressure sensitivematerials in this invention are similar to the polyurethanes. Reactionmixtures are diluted to a solid content of 5% to 10% (weight/volume)using solvents including ethanol, isopropanol, methyl ethyl ketone,acetone, and toluene. The invention may include some or all of the abovelisted solvents (in various ratios in the formulation) to control theevaporation rate and coating qualities. In addition, the formulations inthis invention can be sprayed using conventional air powered sprayingequipment in the range of 15 to 40 psi.

ADVANTAGES OVER THE PRIOR ART

The prior art material with respect to Gouterman exploits thephotochemical process of dynamic quenching by oxygen to vary theemission light intensity with changes in pressure. The reliance upon theoxygen component contributes to the sensitivity of the material. Theprior art material with respect to Kelley et al. exploits photochemicalsystems and focuses on the use of these systems exclusively inpolystyrene, which limits the useful range of application. By using thematerial disclosed herein, Applicant has overcome the limitationassociated with polystyrene without incurring a penalty associated withoxygen. In the preferred embodiment of the invention, photochemicalsystems, i.e. exciplex or fluorescence resonance energy transfer (FRET),are exploited to remove the reliance on oxygen for pressure sensitivity.Both the exciplex and FRET systems provide a rapid response to changesin pressure. In addition, the compressibility of the material with theexciplex and FRET system is reversible. Accordingly, the removal of thereliance on oxygen as a contributor to detecting changes in pressureprovides an improved response time as well as enhances sensitivity inapplication of the material.

ALTERNATIVE EMBODIMENTS

It will be appreciated that, although specific embodiments of theinvention have been described herein for purposes of illustration,various modifications may be made without departing from the spirit andscope of the invention. In particular, other types of distance dependentphotochemical systems or materials used as host matrices or componentsof host matrices may be implemented into the pressure sensitivematerial. Accordingly, the scope of protection of this invention islimited only by the following claims and their equivalents.

1-15. (canceled)
 16. A polymeric material comprising: an elastomer; aphotochemical system including fluorescent distance probing moleculesmodified for incorporation into said elastomer; and said photochemicalsystem comprising ranging from about 1 milligram to about 100 milligramsof fluorescent distance probing molecules per 10 grams of polymer. 17.The polymeric material of claim 16, wherein said photochemical system isselected from the group consisting of: an exciplex and a fluorescenceresonance energy transfer (FRET).
 18. The polymeric material of claim16, adapted to produce an optically detectable response to a change inload.
 19. The polymeric material of claim 30, wherein said polyacrylatecomprises a monomer selected from the group consisting of: butylacrylate and methyl methacrylate.
 20. The polymeric material of claim30, wherein said silicone is selected from the group consisting of: GEsilicone TSE-399c and a high viscosity silicone sealant.
 21. A solutioncomprising the polymeric material of claim 16, adapted for applicationto a surface.
 22. The polymeric material of claim 19, wherein saidacrylate comprises from about 20% to about 90% butyl acrylate by weight.23. The solution of claim 21, comprising from about 3% to about 10% ofsaid elastomer by weight.
 24. The solution of claim 21, comprising asolvent selected from the group consisting of: ethanol, methanol,isopropanol, methyl ethyl ketone, acetone, and toluene.
 25. The solutionof claim 21, comprising a solvent selected to control at least onesolution property selected from evaporation rate, coating thickness, andcoating quality.
 26. The solution of claim 21, adapted to be applied toa surface through an air powered spraying apparatus adapted to applysaid solution under pressure ranging from about 15 psi to about 40 psi.27. The polymeric material of claim 17, said exciplex comprising amolecule combination selected from the group consisting of: anthraceneand dimethylaniline, perylene and dimethylaniline, and pyrene andperylene.
 28. The polymeric material of claim 17, a FRET comprising aFluorescein donor and a Rhodamine B acceptor.
 29. The polymeric materialof claim 18, wherein said change in load is selected from a groupconsisting of: global changes and local changes.
 30. The polymericmaterial of claim 16, where said elastomer is selected from the groupconsisting of polyacrylate and a silicone.